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Sulfate Scum

A yellow coloration can appears on the dry glaze surface, or on the bisque clay surface on the opposite side of a vessel wall that has just been glazed. The coloration does not appear immediately, but some time during the dryout of the piece. This is a product of soluble or partially soluble sulfates in the clay which, when given extended contact with the water from the applied glaze, dissolve and migrate to the surface with it. This phenomenon can happen even with porcelains and it is the product of drying the ware too slowly. In industry, care is take to dry ware quickly so the solution and migration does not occur. This can be done by heating the ware or putting it in a drying chamber immediately after glazing.

A term describing the whitish or brownish dry or glassy scum (depending on iron content and firing temperature) left on the surface of a fired clay body (most often red earthenware or raw stoneware and fireclays). Many clays contain soluble sulphates that migrate to the surface with the water and ar...